1. Field of the Invention
The present invention relates to a three-dimensional image measuring device which obtains three-dimensional coordinate positions of an object without using triangulation.
2. Description of the Related Art
In an active measuring process, light is cast upon on an object, the strength of light scattered on the surface of the object is measured and coordinate positions of the object are measured on the basis of the measured strength values using the principle of triangulation.
This process, however, has the following problems:
1) The distance to the object is required to be measured on the basis of Geometric arrangement of the light source, projection angle of the light and light receiving points (triangulation) and the calculation would be complicated. PA1 2) Since the light source and the light receiving points cannot be arranged on the same axis, an invisible area would be produced. PA1 1) Control is always required to be provided such that the focus of the beam is at a position on the surface of the object. PA1 2) In order to measure the distance with high accuracy, a lens is required which is very shallow in depth of focus, small in aberration, and large in aperture. If the object is at a distance of more than several meters from the measuring device, high measurement accuracy is not obtained.
Published unexamined Japanese patent application Hei 4-1509, entitled "Non-Contact Three-Dimensional Coordinate Measuring Process", discloses a technique which measures without using the principle of triangulation.
This technique relates to a three-dimensional beam scanner capable of focusing a laser beam on any point within the measurement space. The scanner is controlled such that the focus of the beam is at a position on the surface of an object to be measured, at which time the three-dimensional coordinate position of the object is obtained on the basis of a tilt of the laser beam within the reference coordinates and the focal length.
This conventional technique has the following problems:
Apart from the active measurement, there is a passive measurement process for measuring the three-dimensional coordinates of the object by moving the lens toward the object in place of beam irradiation to determine a blur in the image. In this process, a position in an image to which the lens is focused is the focal length of the lens. Thus, the distance from the lens position is determined.
Since, however, this passive measurement process is intended to effect the distance measurement using the lens focus, a lens is required which is very shallow in depth of focus, small in aberration, and large in aperture in order to measure the distance with high accuracy, as in the second conventional technique. If the subject is at a distance of more than several meters from the measuring device, high measurement accuracy is not obtained.